Electric protective apparatus



Dec. 31, 1940. O 2,227,125

ELECTRIC PROTECTIVE APPARATUS Filed April 11, 1940- MOTOR FRAME Inverwtzor: Leonard W. Cook,

by f

His Attorney.

Patented Dec. 31, 1940 UNITED STATES ELECTRIC PROTECTIVE APPARATUS Leonard W. Cook, Stratford, Conn., assignor to General Electric Company, a corporation of New York Application April 11,

4 Claims.

My invention relates to an electric protective apparatus for circuit interrupters and more particularly where such circuit interrupters are used in connection with motors of the commutator type.

Wherever electric switches are employed, the question of suppressing and extinguishing arcs accompanying the operation of the switches arises. The interruption of alternating-current circuits is readily accomplished because the arc or discharge occurring upon the separation of the switch contacts tends to extinguish itself automatically when the alternating current passes through the zero point of the current wave. However, in a direct-current circuit, a different situation obtains for there is no tendency for the arc to extinguish itself since the current never goes through zero and, therefore, the interrupting ability of a given switch when used in connection with such direct-current circuits is generally very much less than when used in alternating-current circuits.

Many of the modern appliances are provided with universal motors to operate either from a 25 direct-current or alternating-current source. In apparatus of this type, it has been necessary to incorporate an expensive switch of the high-speed break type so that the circuit may be interrupted properly in the event that the apparatus is operated from a direct-current source, whereas, if it were only to be operated from an alternatingcurrent source, a much cheaper switch of the slow-break type could be utilized. It would be desirable, therefore, to have a switching device for interrupting direct-current circuits which would cost approximately the same as one for interrupting alternating-current circuits while still having the same interrupting capacity.

Arrangements utilizing arc quenching capacitors have been proposed whereby the cheaper slow-break type of switches could be used effectively to interrupt direct-current circuits. My invention relates to such an arrangement as is disclosed and claimed in copending joint application Serial No. 321,048, filed February 27, 1940, on behalf of C. G. Suits and A. M. Gurewitsch and assigned to the same assignee as the present invention.

Many of the so-called universal appliances which may be used on either an alternatingcurrent or a direct-current source and, therefore, include commutator motors, must be provided with filters of some sort for the suppression of radio interference and these filters usually comprise condenser combinations. In accordance 1940, Serial No. 329,120

with my invention, I utilize these condensers not only to eliminate high-frequency currents which cause radio interference but also to aid in extinguishing the arcs which accompany the operation of associated electric switches whereby a 5 slow-break electric switch may be used whether the appliance is energized from a source of direct or a source of alternating current.

It is an object of my invention, therefore, to provide a new and improved electric protective 10 device for suppressing arcs in electric switches used in connection with commutator motors so as to adapt switches to interrupt more severe arcs than can be extinguished by mere separation of the contacts and wherein this same apparatus 15 cooperates to minimize the radio influence of said motors.

Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty 20 which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

For a better understanding of my invention, reference may be had to the accompanying draw- 25 ing the single figure of which is a schematic diagram of a circuit embodying my invention.

My invention is applicable to any load circuit of the type which produces high-frequency currents having a radio influence which may be 30 energized either from a source of direct or alternating current. Accordingly, I have illustrated my invention as applied to a multispeed commutator motor IO preferably of the type adapted to operate on either alternating current or direct current from source H which may be either an alternating-current or direct-current source. The motor I 0 may be controlled by means of a suit able switch l2 which I have illustrated in the drawing as of the multipcsition type whereby the 40 speed of electric motor I0 may be varied. The details of the construction of this switch form no part of the present invention and it has been deemed unnecessary to illustrate in detail its specific structure. As will be described in greater detail hereinafter, switch I2 is preferably of the slow-break type since such switches from an economy standpoint are more desirable.

Commutator motor In is provided with series field windings l3 and M which should be dis- 5 tributed more or less symmetrically on the yoke of the motor since it has been found that this type of distribution aids in suppressing radio interference with a minimum of filter expense. With only one field coil connected to one terminal 5 of the armature lUa of electric motor ID, a certain amount of radio interference is produced which cannot be eliminated unless an excessive amount of filter capacitance is used. In order to control the speed of motor I0, field winding I3 is provided with a plurality of taps which are connected to contacts l2a, lib, and I20, of switch l2. Switch l2 controls the energization of motor field l3 so that, when the switch is in the position 12a, the motor will operate on maximum field and thereby operate at minimum speed; when in its position I21), the motor will operate on reduced field and, therefore, will operate at increased speed; and when in its position I20 the motor will be operating on minimum field strength and, therefore, at its maximum speed. In other words, three speeds are provided, known as low, medium, and high speeds.

Since motor I!) has been illustrated as of the commutator type, it is provided with arcing contacts, namely, the brushes Ill which produce high frequency components of current and voltage giving rise to radio interference. In order to minimize the radio influence of motor in, I provide a filter l5 including condensers or capacitors l6, l1, and I8 which provide substantially a short-circuit path for the high-frequency currents produced by motor so that practically none of these currents are reflected in the circuit between motor In and source ll. Capacitors l6 and H connected across the motor terminals provide a very low impedance for the balanced highfrequency currents which might cause radio interference and, in order to eliminate the effect of any unbalanced high-frequency currents, the midpoint I9 between capacitors l6 and I1 is connected to the motor frame, schematicallyshown, through a low-impedance path including capacitor H3. The capacitance of condenser I8 is preferably somewhat smaller for best results than that of condensers l6 and I1. By means of filter l5, therefore, a low-impedance path has been provided for these high-frequency currents so that they are eliminated from the electric circuit interconnecting motor In with the source of power II.

In the copending joint application mentioned above is described an arrangement wherein a large increase in the interrupting capacity of a switch is obtained if a condenser charged to a polarity opposite to that of the potential across the switch contacts when the circuit is interrupted is suddenly applied across said switch contacts after a predetermined separation thereof. The results of tests set forth in this joint application further show that, for a given arc-quenching condenser and a given applied voltage, there is an optimum contact separation which should exist at the instant the reversely or oppositely charged condenser is connected across the switch contacts in order to obtain maximum interrupting ability.

In accordance with my invention, condenser it of the filter l for minimizing the radio influence of motor I0 is arranged so as to be charged to a potential opposing the potential across the contacts of switch l2 and to be connected across the contacts of switch l2 after a predetermined separation thereof for arequenching purposes whereby switch l2 may be of the slow-break type and, therefore, considerably cheaper than would ordinarily be suitable when source H is a direct-current source of potential. Switch i2 is illustrated as being provided with an additional contact l2 located at a predetermined distance, as set forth in the above-mentioned joint application, from contact I2c which distance is dependent upon the size of condenser l6 and the potential of the source H. Condenser l6 may be charged to a predetermined potential, which opposes the potential across the contacts of switch l2 upon interruption of the circuit, through condenser H which provides a high-impedance charging path. However, as illustrated in the drawing, I may also provide a resistor having a high impedance connected in series with condenser l6 across source II when switch I2 is closed to energize motor I0. It will be understood by those skilled in the art that, when switch I2 is associated with contact l2c, condenser l6 will be reversely charged to substantially one half the potential of source I I. An appreciable charging time is, of course, required for condenser l6 but, in practical cases, the values of capacitance and resistance of condenser l6 and resistor 20, respectively, are such that this charging time is negligibly small in comparison to the period determined by the frequency of operation of the switch.

The operation of my electric protective apparatus will be obvious to those skilled in the art. Filter l5 will substantially suppress the highirequency currents generated by commutator motor l0, thereby minimizing the radio influence thereof. At the same time, condenser l6 of this filter is arranged to be reversely charged relative to the potential across switch 12 upon interruption of the circuit so as rapidly to quench the associated are drawn, when suddenly applied across the switch contacts after a predetermined separation thereof, thereby greatly increasing the interrupting capacity of switch l2. By this arrangement, a current zero is effectively obtained even though motor ill is energized from a source of direct current.

I have, therefore, provided a simple and very compact circuit-interrupting device for a commutator motor which may be energized either from a direct-current or an alternating-current source wherein the radio infiuence of the commutator motor is minimized by the same apparatus which increases the interrupting capacity of the circuit-interrupting device thereby serving in a dual function.

It should be understood that my invention is not limited to the specific details and arrangement thereof herein illustrated and I intend in the appended claims to cover all such changes and modifications as fall within the spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is 1. In combination, a source of electrical energy, a load circuit of the type which produces high-frequency currents having a radio influence, a switch including a plurality of contacts for connecting and disconnecting said source and said load circuit, a filter for minimizing the radio influence of said load circuit, said filter including a condenser, and means for shunting said condenser across said switch contacts during said disconnecting operation in order to increase the current-interrupting ability of said switch.

2. In combination, a source of electrical energy, a load circuit of the type which produces high-frequency currents having a radio influence, a switch including a plurality of contacts for connecting and disconnecting said source and said load circuit, a filter for minimizing the radio influence of said load circuit, said filter including a condenser arranged to be charged to a potential having a polarity opposite to the polarity of the potential across said switch contacts when disconnecting said source and said load circuit, and means for shunting said reversely charged condenser across said switch contacts during said disconnecting operation after said contacts have been separated to a predetermined distance to increase the current-interrupting ability of said switch.

3. In combination, a source of electrical energy, a commutator motor, a switch including a plurality of contacts for connecting and disconnecting said source and said motor, a filter for minimizing the radio influence of said motor, said filter including a condenser arranged to be charged to a potential having a polarity opposite to the polarity of the potential across said switch contacts when disconnecting said motor from said source, and means for shunting said reversely charged condenser across said switch contacts during said disconnecting operation to increase the current-interrupting ability of said switch.

4. In combination, a source of direct current, a multispeed motor of the type which produces high-frequency currents having a radio influence, a switch for connecting and disconnecting said motor and said source and controlling the seed of said motor including a plurality of contacts, a filter for minimizing the radio influence of said motor, said filter including a condenser arranged to be charged to a potential having a polarity opposite to the polarity of the potential across said switch contacts when disconnecting said motor from said source, and means for shunting said reversely charged condenser across said switch contacts during said disconnecting operation after said contacts have been separated to a predetermined distance whereby the currentinterrupting ability of said switch is greatly increased.

LEONARD W. COOK. 

